# Millimeters to inches converter

> Convert millimeters to inches (mm to inches) with the exact factor, a conversion chart and worked steps.

Interactive version: https://www.calcopenly.com/conversion/mm-to-inches
Subject: Unit converters

Millimeter readings are useful for small clearances, fasteners and drawing dimensions. Retain the tolerance with the original measurement: converting the number cannot make a manufactured part more accurate. Check whether a nominal part size describes its actual measured diameter.

One millimetre is about 0.0393700787 inches, so 25 mm is 0.984252 in. To convert mm to inches, multiply by 0.0393700787; to convert back, multiply by 25.4.

The millimetre is one thousandth of a metre, the SI base unit of length. The factor has no exact decimal form, so results are computed with 50 significant digits and rounded only for display.

## Inputs

- **Millimeters (mm)**

## Results

- Inches (in) — main result
- Conversion
- 1 in in millimeters (mm)

## Formula

$$
\text{inches} = \text{millimeters} \times 0.0393700787402
$$

## Worked examples

### 25 mm to in

- Millimeters (mm): 25 mm
- **Inches: 0.9842519685 in**
- Checked against: Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions)

### 8 mm to in

- Millimeters (mm): 8 mm
- **Inches: 0.3149606299 in**
- Checked against: Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions)

### 50 mm to in

- Millimeters (mm): 50 mm
- **Inches: 1.968503937 in**
- Checked against: Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions)

## Questions

### When would I convert mm to inches?

Millimeter readings are useful for small clearances, fasteners and drawing dimensions. Retain the tolerance with the original measurement: converting the number cannot make a manufactured part more accurate. Check whether a nominal part size describes its actual measured diameter.

### How many inches are in a millimetre?

One millimetre is about 0.0393700787 inches. The millimetre is one thousandth of a metre, the SI base unit of length.

### How do you convert mm to inches?

Multiply the number of millimeters by 0.0393700787. For example, 25 mm × 0.0393700787 = 0.984252 in. To go the other way, divide by 0.0393700787.

### How can I estimate mm to inches without a calculator?

Multiply by 0.04. That is 1.6% high of the exact factor 0.0393700787, so 25 mm comes out as 1 in instead of 0.9843.

### How accurate is the millimeters to inches converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “25 mm to in” is checked against Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions).

### Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B; NIST Handbook 44, Appendix C: General tables of units of measurement; BIPM, The International System of Units (SI Brochure), 9th edition.

## Sources

- [NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C: General tables of units of measurement](https://www.nist.gov/pml/weights-and-measures/publications/nist-handbooks/handbook-44)
- [BIPM, The International System of Units (SI Brochure), 9th edition](https://www.bipm.org/en/publications/si-brochure)